Engines used in space flight and engines for driving underwater devices, such as a torpedo, require a fuel that can be combusted independently of atmospheric oxygen. Such fuels usually comprise a further component functioning as an oxidizer and such further fuel component is usually present in liquid form. Fuels of this type include a solid fuel component in the form of a metal hydride and an inert liquid component, preferably an alkane in which the solid fuel component is embedded. When the solid fuel component is a hydride, it is preferably a hydride of the alkali metals or a metal powder embedded in a binder.
However, fuels for the above purpose are also known in the art in which the fuel component is provided in liquid form for example, as a hydrocarbon, such as hydrazine. Cryogenic hydrogen has also been used for this purpose in liquid form.
While the solid fuels have the advantage of a high energy density, the liquid fuels have the advantage of generally being more efficiently handled, particularly conveyed and controlled. In order to achieve the advantages of both types of fuel simultaneously, it has been suggested to use an alkali metal hydride, for example lithium hydride (LiH), which is embedded in granular form in a liquid paraffin (C.sub.10 H.sub.20), whereby the resulting product has a paste-type viscous form. Problems have been encountered in practice with this type of viscous fuel because of the characteristic behaviour of alkali metal hydrides which tend to become instable, especially due to a premature release of hydrogen.